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AAPG Bulletin, V.
An eolian facies within the Upper Jurassic Smackover Formation, Tchula Lake field, Mississippi
1Department of Geology and Geological Engineering, University of Mississippi, Oxford, Mississippi 38677; email: [email protected]
2Department of Geology and Geological Engineering, University of Mississippi, Oxford, Mississippi 38677; email: [email protected]
3Jura-Search, Inc., P.O. Box 997, Jackson, Mississippi 39205; email: [email protected]
William Tedesco received a B. S. degree in geology and geophysics from the University of Missouri-Rolla in May 1997. During the summer of 1997, he worked as an intern for Western Geophysical on a transition zone three-dimensional seismic crew in coastal Louisiana. He received a master's degree in geology from the University of Mississippi in 1999. He is currently pursuing a Ph.D. in geology at the University of Mississippi, specializing in carbonate sedimentology and diagenesis, stratigraphy, and reservoir modeling.
R. P. Major holds a Ph.D. in geology from Brown University and was a National Research Council Postdoctoral Associate with the U.S. Geological Survey in Denver. He has held several university teaching and/or research positions, notably at the University of Texas at Austin, the University of Colorado at Denver, and Union College. He is a former senior geologist with Amoco Production Company, and he has served on the editorial boards of the Journal of Sedimentary Research and the AAPG Bulletin. Since 1998 he has been professor and chairman of the Department of Geology and Geological Engineering at the University of Mississippi.
Lawrence Baria, after receiving a B.S. degree in geology in 1967, pursued a master's degree in stratigraphy and sandstone petrology at Northeast Louisiana University and Ph.D. studies in carbonate facies, stratigraphy, and sulfate diagenesis at Louisiana State University (LSU). After leaving LSU, he went to work for Getty Oil Company in their E & P Research lab, specializing in Cretaceous and Jurassic stratigraphy. Since 1980 he has been a consulting and exploration geologist active in the Haynesville, Smackover, and Norphlet formations throughout the central and eastern Gulf Coast.
We thank the Gulf Coast Association of Geological Societies for providing funding for this project in the form of a student research grant to W. A. Tedesco. The research was supported as part of Tedesco's master's thesis at the University of Mississippi under the direction of R. P. Major. We thank Clyde H. Moore and Mitch Harris for comments that greatly improved this article. Thanks also to Ezat Heydari for discussions and assistance that helped clarify this work.
The Late Jurassic upper Smackover Formation in Mississippi is primarily composed of an overall regressive sequence of marine carbonate ramp deposits. In central Mississippi, however, carbonate deposition was interrupted by a large influx of siliciclastic sediment. Tchula Lake field is unique not only because it produces gas and condensate from dolomitic quartz sandstone deposited in a mixed siliciclastic-carbonate environment but also because it produces from what we interpret to be an eolian dune sand facies. Throughout the Gulf Coast Basin no previous literature has cited an eolian facies within the Smackover Formation. Several characteristics of the producing zone sandstones have led to interpretations of this unit as an eolian coastal dune facies deposited on and around a Jurassic paleohigh.
Facies and paleoenvironmental interpretations are based on compositions and textures described in cores and thin sections. The eolian facies consists of fine-grained, dolomitic quartz sandstone with a mean porosity of 10-20% and permeability of 0-800 md. The sand grains are rounded to subrounded and well sorted. The sandstone exhibits a sequence of upward-steepening planar laminations capped at the top by a set of small-scale wedge planar cross-laminations. The sharply defined, parallel lamina are less than 5 mm thick and are commonly inversely graded and contain probable vadose meniscus carbonate cements. This facies is interpreted to have been deposited in a backshore coastal dune environment. The unit is characterized by low-angle (<20 degrees dip) climbing translatent strata commonly produced by the migration of wind ripples along a dune. No fossils or trace fossils are found within this facies.
Eolian-deposited sandstone is a potential reservoir rock along the updip limit of the Smackover Formation in west-central Mississippi. Other paleohighs along the Jurassic shoreline may have undergone similar eolian deposition during a regional sea level fall. Eolian-deposited sandstones on these paleohighs should have (Begin page 486) similar reservoir properties as the sandstone at Tchula Lake field. The updip Smackover Formation in west-central Mississippi is an underexplored horizon, and eolian-deposited reservoirs have strong potential as a new exploration target. The sea level fluctuation event associated with deposition at Tchula Lake field also has implications for exploration in Upper Jurassic sedimentary rocks across the entire United States Gulf Coast Basin and prompts a reevaluation of these reservoirs.
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